Surface Sites Of TiO2 Research Articles

Photo-oxidation of nitrite anions in aqueous solution for the benchmarking of nano-TiO2 photocatalytic coatings

Photocatalytic coatings have potential as an air purifying technology for removal of pollutants such as NOx gases, which are converted into and immobilized as nitrate via photo-oxidation. This work is an investigation of the photocatalytic conversion of nitrite into nitrate in the aqueous phase. The study of this reaction is important to understand the mechanism through which the immobilization of the NOx species as nitrates takes place during NOx conversion, especially in humid environments. We investigate the photocatalytic performance of TiO2 nanoparticles (Aeroxide® P25) and the corresponding photocatalytic coatings prepared using acrylic binder (TiO2 NP-acrylic) towards the photo-conversion of nitrite. The combination of a simple colorimetric method (Griess test) for nitrite quantification and UV–vis spectroscopy for nitrite and nitrate analysis is shown to be a useful method for monitoring the nitrite conversion. Photocatalytic tests with suspended TiO2 nanoparticles show that the nitrite conversion benefits from lower pH, higher temperature and higher concentration of the photocatalyst. The advantage of lower pH is attributed to electrostatic attraction between TiO2 with adsorbed H3O+ and NO2−, which facilitates transport of nitrite to the active surface. When the photocatalyst is immobilized in a coating the factor of primary significance is the pigment volume concentration relative to critical pigment volume concentration (λ), as the nitrite conversion increases linearly with λ in the range 0.6–1.2. This is attributed to the creation of a more porous structure that facilitates the access of nitrite and light to the active TiO2 surface sites.

Understanding the intrinsic synergistic mechanism between Pt-O-Ti interface sites and TiO2 surface sites of Pt/TiO2 catalysts in Fenton-like reaction

Understanding the intrinsic synergistic mechanism between Pt-O-Ti interface sites and TiO2 surface sites of Pt/TiO2 catalysts in Fenton-like reaction

Atomistic insights into uptake of hydrogen peroxide by TiO2 particles as a function of humidity

In this work, a multiscale and comprehensive study for the heterogeneous interaction between H2O2 and TiO2 particles has been explored. It is found that the H2O2 and H2O adsorption on the TiO2 surface mainly depends on the weaker interaction between surface Ti atom and O atom in H2O2 and H2O. The calculation results indicated that the adsorption energy was dominated by electrostatic effect. Besides, the adsorption energy of TiO2-H2O complexes is smaller than that of TiO2-H2O2 ones. We also found that the interaction of H2O2 and H2O on TiO2 particles is displayed by molecular adsorption and dissociation. It will modify the particle surface and results in the formation of Ti-OH. The reaction sites came from the Ti and O atom on TiO2 particle surface. After the reaction, the surface Ti atoms were oxidized. Moreover, H2O2 uptake on TiO2 surface was suppressed with the increase of humidity likely by H2O could competition for TiO2 surface sites. Based on the findings report here, the heterogeneous reaction between H2O2 and TiO2 particles was investigated at atomic-level, which can be help for understanding the effect of modified mineral dust on human health.

Simultaneous arsenic and fluoride removal using {201}TiO2-ZrO2: Fabrication, characterization, and mechanism.

The coexistence of arsenic (As) and fluoride (F) in drinking water is an urgent environmental issue that causes increasing public concerns. The need for effective simultaneous removal of As and F has motived great research efforts. Herein, a novel {201}TiO2-ZrO2 composite was synthesized and its application mechanism was explored. Batch adsorption experiments show that the As(III), As(V), and F adsorption followed the pseudo-second-order kinetics with the Langmuir adsorption capacity at 58.5, 21.6, and 13.1 mg/g, respectively. EXAFS and in situ ATR-FTIR results suggested that TiO2 surface sites were occupied by As(III) and As(V) in bidentate binuclear structures, and ZrO2 sites preferentially adsorbed As(III) and F in monodentate mononuclear configurations. This molecular structure obtained in the mono-adsorption system was integrated with the charge distribution multisite surface complexation model to accurately predict the As and F co-existing adsorption behaviors. The results in competitive adsorption, regeneration, and application evidenced that the {201}TiO2-ZrO2 composite is a promising adsorbent for simultaneous As and F removal.

Groundwater Arsenic and Silicate Adsorption on TiO2 and the Regeneration of TiO2

Titanium dioxide (TiO2) was widely used to remove arsenic (As) from groundwater due to its excellent properties. Previous studies show that the coexisting silicate ions (Si) could occupy the available surface sites of TiO2 and further inhibit As adsorption and TiO2 regeneration. To investigate the effect of Si adsorption on the As molecular surface structure, an extended X-ray absorption fine structure (EXAFS) analysis was conducted in this work. The results indicated that the presence of Si exhibited no impact on the As adsorption configuration on TiO2. In situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy results demonstrated that the polymerization of Si that formed on the TiO2 surface compete with As adsorption sites, increasing the difficulty for TiO2 regeneration. To effectively regenerate TiO2, the removal efficiency of Si polymers on TiO2 via sodium fluoride (NaF) was studied. The results showed that NaF could remove Si monomer and polymer from TiO2, and the regenerated TiO2 could be reused with a stable adsorption performance. In situ ATR-FTIR spectroscopy suggested that NaF desorbed the Si monomer and polymer effectively. When spent TiO2 was regenerated with NaOH and NaF in three treatment cycles, As and Si desorption rates were 86.8%-100.3% and 67.9%-82.0%, respectively. The present study provides a new insight into regenerating absorbents with coadsorbed As and Si in groundwater.

Reduction of adsorbed As(V) on nano-TiO2 by sulfate-reducing bacteria

Reduction of surface-bound arsenate [As(V)] and subsequent release into the aqueous phase contribute to elevated As in groundwater. However, this natural process is not fully understood, especially in the presence of sulfate-reducing bacteria (SRB). Gaining mechanistic insights into solid-As(V)-SRB interactions motivated our molecular level study on the fate of nano-TiO2 bound As(V) in the presence of Desulfovibrio vulgaris DP4, a strain of SRB, using incubation and in situ ATR-FTIR experiments. The incubation results clearly revealed the reduction of As(V), either adsorbed on nano-TiO2 or dissolved, in the presence of SRB. In contrast, this As(V) reduction was not observed in abiotic control experiments where sulfide was used as the reductant. Moreover, the reduction was faster for surface-bound As(V) than for dissolved As(V), as evidenced by the appearance of As(III) at 45h and 75h, respectively. ATR-FTIR results provided direct evidence that the surface-bound As(V) was reduced to As(III) on TiO2 surfaces in the presence of SRB. In addition, the As(V) desorption from nano-TiO2 was promoted by SRB relative to abiotic sulfide, due to the competition between As(V) and bacterial phosphate groups for TiO2 surface sites. This competition was corroborated by the ATR-FTIR analysis, which showed inner-sphere surface complex formation by bacterial phosphate groups on TiO2 surfaces. The results from this study highlight the importance of indirect bacteria-mediated As(V) reduction and release in geochemical systems.

Polymerization of silicate on TiO2 and its influence on arsenate adsorption: An ATR-FTIR study

The exposure risks of toxic arsenic in the environment are dependent on the adsorption and desorption processes at solid/aqueous interfaces, which could be greatly influenced by co-existing oxyanions such as silicate. In the present work, the adsorption of arsenate [As(V)], silicate (Si), and the influence of silicate on As(V) adsorption are investigated at the molecular level by in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Silicate monomer, dimer, oligomer, and polymer were detected on the TiO2 surface after 3 h, and silicate oligomer was the predominant surface species under the experimental conditions. The presence of silicate with different silicate and As(V) adsorption sequences did not interfere with As(V) surface species on TiO2, but only affected the As(V) adsorption capacity. For silicate, the adsorption of As(V) could greatly inhibit the formation of Si monomer and dimer on TiO2, but the silicate oligomer and polymer were not influenced by As(V) adsorption. The silicate oligomeric and polymeric surface species will gradually occupy the TiO2 surface sites to inhibit As adsorption and TiO2 regeneration. Our findings provide spectroscopic evidence and clearly elucidate the molecular-level interactions of silicate and As(V) on TiO2, which will further our understanding of the removal of As in groundwater. (C) 2015 Elsevier B.V. All rights reserved.

Characterization of Cr Sorption and Reduction on TiO2: Batch and XPS Studies

Cr interactions with TiO2 were systematically studied using batch and spectroscopic investigations. Sorption of chromium on TiO2 at pH 4.5 increases with increasing Cr concentration. The sorption of Cr(III) is in good agreement with Langmuir isotherm model, whereas that of Cr(VI) is better accounted for by the Freundlich model. At pH 7.0, however, the uptake of Cr(III) by TiO2 is over 95 %, while the extent of Cr(VI) sorption on TiO2 is much less than that of pH 4.5. These results are consistent with SEM observations showing that precipitates of Cr(III) are dominant under neutral pH. The sorption of Cr(VI) on TiO2 decreases with increasing pH. However, Cr(VI) sorption decreases with increasing ionic strength below pH 4.5 whereas the sorption increases with ionic strength above pH 4.5. These observations suggest that Cr(VI) sorption is sensitive to ionic strength, and Cr(VI) could form weakly bound adsorption complexes at the TiO2–water interface. Phosphate competes with Cr(VI) for TiO2 surface sites during sorption processes, and Cr(VI) desorption accelerates and increases in the presence of phosphate. It is noted that the reduction of Cr(VI) is induced by sunlight on the TiO2 surface, but not detected in acidic solution throughout batch experiments at pH ≥ 4.5 for 24 h.

Lewis-Acid Sites of TiO2 Surface for Adsorption of Organic Dye Having Pyridyl Group as Anchoring Unit

Adsorption of an organic dye having a pyridyl group as an anchoring unit (NI4) is studied on nanocrystalline TiO2 surface by being compared with a similar dye having a carboxyl anchor (NI2). Adsorption of both dyes followed a Langmuir isotherm, and analysis of the isotherms showed that an adsorption equilibrium constant of NI4 is (1.1 ± 0.1) × 103 M–1, much smaller than (0.6 ± 0.05) × 105 M–1 for NI2, and amounts of adsorbed dyes at saturation are similar to each other: [(1.5 ± 0.2) and (1.7 ± 0.3)] × 1014 cm–2 for NI2 and NI4, respectively. Coadsorption experiments with 4-carboxy TEMPO (4CT) and either NI2 and NI4 revealed that the adsorption sites of NI2 and NI4 are different from each other. This was supported by measurements of average nearest-neighbor interspin distances of 4CT radicals coadsorbed on TiO2 by a spin-probe ESR technique. The above findings and elaborate FT-IR studies demonstrated that NI2 and NI4 adsorb, respectively, on Bronsted- and Lewis-acid sites of TiO2 surface. The number of Lew...

Fabrication of Al-Doped TiO2Visible-Light Photocatalyst for Low-Concentration Mercury Removal

High-quality Al-doped TiO2visible-light photocatalyst was prepared via a single-step direct combination of vaporized Ti, Al, and O2using a 6 kW thermal plasma system. Results showed that the formed Al-doped TiO2nanoparticles were a mixture of anatase and rutile phase and had a size between 10 and 105 nm. The absorption spectra of the nanoparticles shifted towards the visible light regions, depending on the Al2O3addition. Ti4+and Ti3+coexisted in the synthesized Al-doped TiO2; the Ti3+concentration, however, increased with increasing Al2O3addition due to Al/Ti substitution that caused the occurrence of oxygen vacancy. Hg0breakthrough tests revealed that the nanoparticles had an appreciable Hg0removal under visible-light irradiation. Nevertheless, moisture reduced Hg removal by the nanoparticles, especially when visible-light irradiation was applied, suggesting that the competitive adsorption between H2O and Hg species on the active sites of TiO2surface occurred.

The role of poly(methacrylic acid) conformation on dispersion behavior of nano TiO2 powder

To exploit the advantages of nanoparticles for various applications, controlling the dispersion and agglomeration is of paramount importance. Agglomeration and dispersion behavior of titanium dioxide (TiO2) nanoparticles was investigated using electrokinetic and surface chemical properties. Nanoparticles are generally stabilized by the adsorption of a dispersant (polyelectrolyte) layer around the particle surface and in this connection ammonium salt of polymethacrylic acid (Darvan C) was used as dispersant to stabilize the suspension. The dosages of polyelectrolyte were optimized to get best dispersion stability by techniques namely particle charge detector (13.75mg/g) and adsorption (14.57mg/g). The surface charge of TiO2 particles changed significantly in presence of dispersant Darvan C and isoelectric point (iep) shifted significantly towards lower pH from 5.99 to 3.37. The shift in iep has been quantified in terms of free energy of interaction between the surface sites of TiO2 and the adsorbing dispersant Darvan C. Free energies of adsorption were calculated by electrokinetic data (−9.8 RT unit) and adsorption isotherms (−10.56 RT unit), which corroborated well. The adsorption isotherms are of typical Langmuir type and employed for calculation of free energy. The results indicated that adsorption occurs mainly through electrostatic interactions between the dispersant molecule and the TiO2 surface apart from hydrophobic interactions.

The influence of surface fluorination in the photocatalytic behaviour of TiO2 aqueous dispersions: An analysis in the light of the direct–indirect kinetic model

TiO2 surface fluorination is known to produce an enhancement of the photocatalytic oxidation rate of some organic compounds such as phenol, acid orange, azo dye Acid Red 1 and benzoic acid, while leads to a clear decrease of the photooxidation quantum yield of other compounds like formic acid, and dichloroacetate. Here we show that these differences in behaviour cannot be explained in the framework of the classical “Langmuir–Hinshelwood” (L–H) kinetic model, although they are compatible with the predictions of the alternative, recently developed “Direct–Indirect”(D–I) model [19]. For weak electronic interaction of the TiO2 surface with dissolved substrate species, as it is for instance the case for phenol, the D–I model predicts that photooxidation takes place mainly via an interfacial, indirect transfer (IT) mechanism involving inelastic trapping of valence band free holes by terminal, twofold coordinated oxygen ions, and further adiabatic transfer to dissolved substrate species. On the basis of open-circuit voltage measurements, the origin of the photooxidation quantum yield increase observed in these cases when the TiO2 surface becomes fluorinated is attributed to the partial substitution of water molecules adsorbed on terminal Ti sites by fluoride ions, leading to: (1) an interfacial decrease of the electron-hole recombination rate as the surface concentration of recombination centres associated with terminal Ti sites diminishes; (2) an increase of the IT rate of surface trapped holes to reduced dissolved species, because of the upward shift experimented by the semiconductor energy levels with respect to filled electrolyte energy levels. In contrast, for strong electronic interaction of substrate species with the naked TiO2 surface, as it is for instance the case of formate ions at low enough pH, photooxidation mainly take place via an interfacial, direct transfer (DT) mechanism, involving inelastic trapping of valence band free holes by specifically adsorbed substrate species. The photooxidation quantum yield decrease observed in this case is explained to be due to a diminution of the density of TiO2 surface sites active for adsorption of formate ions. The enhanced photogeneration of free OHS radicals and/or photooxidation rate of water molecules in the liquid phase with free VB holes, assumed by some authors as the origin of the observed photooxidation quantum yield increase at the fluorinated TiO2 surface, should be disregarded as far as both processes appears to be thermodynamically and kinetically hindered.

Reactivity of the Oxygen Sites in the V2O5/TiO2 Anatase Catalyst

We present for the first time a theoretical investigation on the reactivity of the different oxygen sites in a dispersed V2O5/TiO2 slab model. Our model represents anhydrous conditions and contains all the potentially active oxygen sites proposed to date in the literature: vanadyl VO, bridging V−O−V, interface V−O−Ti, and surface Ti−O−Ti oxygen sites. Geometric features are analyzed in terms of bond distances and site accessibility. Projected density of states diagrams show a decrease of the band gap for the supported system with respect to pure anatase (001). The most energetic levels of the valence band correspond to the oxygen atoms in the V2O5 unit, although a further decomposition in terms of mono- and dicoordinated oxygen is not obvious. Atomic hydrogen adsorption on the different sites is tracked. The most exothermic systems are those where H adsorbs on the V2O5 oxygen sites due to their higher basicity regarding pure anatase. The heats of adsorption referred to atomic H are V−O−Ti (2.95 eV) > VO (2.60 eV) > V−O−V (2.17 eV) > Ti−O−Ti (2.07 eV). This means that the most reactive sites are those located at the interface between the V2O5 and the TiO2 unit, while the vanadyl VO bonds are more stable and would react at a lower extent in hydrogen atomic adsorption. Relaxation makes the adsorption energy decrease: the support relaxes less when hydrogen adsorbs than for the bare V2O5/TiO2 system. H adsorption forms hydroxyl OH bonds by reduction of the vanadium atom, even when the adsorption site is located on the TiO2 surface sites.

Removal of bromate ion from water using TiO2 and alumina-loaded TiO2 photocatalysts

The effects of pH and the surface charge of photocatalyst on photocatalytic reduction of bromate ions in water were examined by using TiO2 and alumina-loaded TiO2 photocatalysts. The amount of adsorbed bromate ion on TiO2 and the reduction rate of bromate ion by the TiO2 photocatalyst increased with lowering pH. It was suggested that the bromate ion adsorbed predominantly on the positively charged surface sites of TiO2 at lower pH, which thus promotes the rate of reduction. The iso-electric point of the TiO2 photocatalyst was shifted to the higher pH region by loading alumina on the TiO2 surface, which resulted in the increase of the adsorbed amount of bromate on the photocatalyst and the reduction rate of bromate ion by the photocatalyst at neutral pH. It was concluded from these results that the photocatalytic reduction of bromate ion can be promoted by pH control of water or by controlling the surface charge of the photocatalyst by loading alumina without pH control of water.

Adsorption of Pb(II), EDTA, and Pb(II)-EDTA onto TiO2

The adsorption of aqueous Pb(II), EDTA, and Pb(II)-EDTA complexes onto TiO2were studied at both stoichiometric and nonstoichiometric Pb(II)/EDTA concentrations. For Pb(II)-TiO2and EDTA-TiO2, a typical cationic and anionic-type of adsorption was noted, respectively. For 10−3and 10−4M Pb(II)-EDTA systems, near-equal adsorption of Pb(II) and EDTA indicated that the complex adsorbs as a single species. Also, a ligand-type Pb(II)-EDTA adsorption, i.e., decreasing adsorption with an increase in the pH, was noted. Systems with EDTA greater than Pb(II) showed near-zero lead removal; competitive adsorption of EDTA and Pb(II)-EDTA onto TiO2was suggested to cause this effect. For Pb(II) concentrations (5 × 10−4and 10−3M) higher than EDTA (10−4M), significantly higher EDTA adsorption at high pH as compared to individual 10−4M EDTA and 10−4M Pb(II)-EDTA systems was noted. Adsorption modeling was completed employing the geochemical speciation model MINTEQA2 employing the diffuse layer model. Inner-sphere complexation was considered to occur between Pb(II), EDTA, Pb(II)-EDTA, and the TiO2surface sites. Surface complexes used in the modeling includedTi-O-Pb+,Ti-EDTAH2−,Ti-EDTA-Pb−, andTi-O-Pb-EDTA3−. The cationic-type complexation,Ti-O-Pb-EDTA3−, was postulated to explain and model the anomalous EDTA adsorption as noted for Pb(II) > EDTA studies. Results from the present study show that the adsorption behavior in aqueous metal/EDTA systems will change with any variation in the contaminant concentration ratios.

Role of Platinum Deposits on Titanium(IV) Oxide Particles: Structural and Kinetic Analyses of Photocatalytic Reaction in Aqueous Alcohol and Amino Acid Solutions

Photocatalytic reaction at 298 K by platinum-loaded titanium(IV) oxide (TiO2−Pt) particles suspended in deaerated aqueous solutions of 2-propanol or (S)-lysine (Lys) was investigated. The TiO2 catalysts with various amounts of Pt loadings were prepared by impregnation from aqueous chloroplatinic acid solution onto a commercial TiO2 (Degussa P-25) followed by hydrogen reduction at 753 K. The physical properties of deposited Pt, e.g., particle size, surface area, and electronic state, were studied respectively by transmission electron microscopy, volumetric gas adsorption measurement, and X-ray photoelectron spectroscopy as well as infrared spectroscopy of adsorbed carbon monoxide. The increase in Pt amount mainly resulted in an increase of the number of Pt deposits, not of their size. The catalysts were suspended in the aqueous solutions and photoirradiated at a wavelength >300 nm under an argon (Ar) atmosphere. The overall rate of photocatalytic reactions for both 2-propanol and Lys, corresponding to the rate of consumption of these substrates, was negligible without Pt loading, increased drastically with the loading up to ca. 0.3%, and was almost constant or a little decreased by the further loadings. However, the rate of formation of pipecolinic acid (PCA) from Lys was improved gradually with a increase of Pt loading up to ca. 2 wt %. These dependences were discussed as a function of Pt surface area, which is employed as a measure that includes the properties of both number and size of Pt deposits. For the photocatalytic dehydrogenation of 2-propanol, the rate dependence could be interpreted semiquantitatively with the model that only the TiO2 particles loaded with at least one Pt deposit can photocatalyze, but the reaction rate is independent of the number of Pt deposits. Therefore, the overall rate is proportional to the number of Pt-loaded TiO2 particles. On the other hand, for the interpretation of the rate of PCA and H2 productions, the number of Pt deposits on each TiO2 particle had to be taken into account. The efficient production of PCA at higher Pt loadings was attributed to the reduction of a Schiff base intermediate produced via oxidation of Lys with positive holes and subsequent intramolecular condensation at the Pt deposit that is close to the site for the oxidation. Otherwise, photoexcited electrons are consumed for H2 production and the intermediate remains unreduced or undergoes further oxidation. It was suggested that the intermediate produced at the TiO2 surface sites within a distance of several nanometers from the Pt deposit undergoes efficient reduction to PCA. Thus, the importance of the distribution of Pt deposits for the preparation of highly active and selective TiO2−Pt photocatalyst has been clearly demonstrated.